Pseudomonas putida is a rod-shaped bacterium commonly found in soil and water. Its diverse metabolism allows it to break down a wide range of organic compounds, including pollutants, making it a subject of interest for bioremediation. Understanding its colony morphology—the visible characteristics of its growth on a solid surface—is a fundamental practice in microbiology.
General Features of Bacterial Colony Morphology
When bacteria multiply on a solid nutrient source like an agar plate, they form visible clusters called colonies. Each colony originates from a single cell, making all cells within it genetically identical. Microbiologists use a standard set of terms to describe the appearance of these colonies, which aids in the preliminary identification of bacterial species.
These descriptions focus on several observable characteristics:
- Form: The overall shape of the colony, which can be circular, irregular, filamentous (thread-like), or rhizoid (root-like).
- Elevation: The colony’s cross-sectional shape as it rises from the agar, such as flat, raised, convex (dome-shaped), or umbonate (raised with a central bump).
- Margin: The edge of the colony, which can be described as entire (smooth), undulate (wavy), or lobate (lobed).
- Texture: The surface of the colony, which may be smooth, rough, or mucoid (slimy).
- Opacity: The degree of transparency, described as transparent, translucent, or opaque.
- Color: The pigmentation of the colony and whether it diffuses into the surrounding medium.
Typical Colony Morphology of Pseudomonas putida
When grown in a laboratory on a standard nutrient agar, Pseudomonas putida colonies exhibit a recognizable appearance. After an incubation period of 24 to 48 hours, the colonies measure between 2 and 4 millimeters in diameter, and this size increases as the culture ages. The form of the colonies is generally circular, and they have a low convex or dome-shaped elevation.
The margin of a P. putida colony is entire, meaning it has a smooth, even edge. The colonies are usually off-white, cream-colored, or grayish. A distinct characteristic of many strains is producing a fluorescent, greenish-yellow pigment called pyoverdine, especially on specific media like King’s B agar. This pigment can be observed glowing under ultraviolet (UV) light.
The surface of the colonies is characteristically smooth and moist, sometimes described as glistening or butyrous (butter-like). Their opacity is generally translucent to opaque. While this description represents the standard appearance, it is important to recognize that variations can occur.
Variations and Influencing Factors in P. putida Colonies
The typical appearance of Pseudomonas putida colonies is not fixed and can be influenced by environmental and genetic factors. The composition of the culture medium is a significant variable, as different nutrient formulations can alter growth rate, colony size, and texture. For instance, specific media are designed to enhance the production of its fluorescent pigments.
Incubation conditions, such as temperature and time, also play a part. P. putida has an optimal growth temperature between 25°C and 30°C, and temperatures outside this range can affect the size and appearance of colonies. As colonies age, their characteristics change, with size increasing and colors potentially becoming more pronounced.
Genetic differences between strains of P. putida are another source of variation. Some strains naturally produce more mucoid colonies due to higher production of exopolysaccharides (EPS), which are slime-like substances. Spontaneous mutations can also lead to observable changes, such as a shift from a smooth to a rough colony texture.
Importance of Studying P. putida Colony Morphology
Observing colony morphology is a quick, preliminary step toward identification. While colony appearance alone is not enough for a definitive identification, which requires further biochemical or genetic testing, it offers initial clues. These clues help distinguish P. putida from other microorganisms on a culture plate.
Visual inspection is also a method for assessing the purity of a bacterial culture. If all colonies on a plate exhibit a uniform appearance consistent with P. putida, it suggests the culture is pure. The presence of colonies with different morphologies is an indicator of contamination by other microbes.
For researchers and industrial applications, monitoring colony morphology is a way to check for strain stability. A sudden change in appearance, such as the loss of pigment production or a shift in texture, can signal that a genetic mutation has occurred. This is important in fields like bioremediation, where consistent bacterial behavior is necessary for reliable performance. Specific colony variants have also been linked to different behaviors, such as biofilm formation.